Vigorous studies have been given on techniques to increase the capacity of optical-fiber transmission with optical fibers.
It is believed that a growth of optical transmission capacity requires the optical fibers for the optical transmission to enable single-mode transmission at the operating wavelength, because the groupings of different speeds of optical signals in various modes can induce mode dispersion inevitably in the propagation through an optical fiber. As a result, the signal waveforms can decay or warp.
Consequently, the single-mode fiber (SMF) was started in use, having a zero-dispersion wavelength around 1.3 μm. At the zero-dispersion wavelength, the fiber was able to have a transmission distance of scores of kilometers, and a transmission capacity of hundreds of Mbps (megabits per second).
In the meantime, the least transmission loss in optical fiber takes place at 1.55 μm of wavelength, where a dispersion-shifted fiber (DSF) with a zero-dispersion wavelength of 1.55 μm or thereabout was developed. This optical fiber enabled the optical transmission optical transmission with a capacity of several gigabits per second around 1.55 μm of wavelength. The same single-mode fibers were laid in long-distance optical transmission routes each with a capacity of several G bit/s in a 1.55 μm wavelength band.
In the latter half of the 1980s it was discovered that transmission loses would increase in an optical fiber in which hydrogen molecules, from a hydrogen gas (H2) trapped in the cable, had been broken. On analysis, the loss increase was assignable to absorption peaks in the transmission-loss spectrum, which hydrogen molecules had induced in the optical fiber. Hydrogen-induced absorption peaks emerge around 1.24 μm, and at 1.52 μm and on the longer-wavelength side. The absorption peaks at 1.52 μm and the longer wavelength were seen to have an adverse impact on the optical transmission around 1.55 μm, firsthand, for instance, as described in ECOC '86, pp 7-10, by Ogai et al.
Concurrently, in terms of 1.31 μm transmission SMF and 1.55 μm transmission DSF, assorted R&D approaches were made to prevent the hydrogen-induced loss increase, from the aspect of fabrication technique or fiber coating material. For example, optical-fiber cables for terrestrial application were usually filled with a filling compound so as to reduce the amount of trapped gaseous hydrogen. Accordingly, no hydrogen-proofing techniques were explored (see e.g., Bellcore-GR-20-Core issue 2, Jul. 1998, Section 6.6.9).
In the recent years, in search for more capacities of optical transmission systems, the designs of wavelength division multiplexing (WDM) have been studied and developed, producing volumes of reports on optimizing optical fibers for WDM transmission.
From the angle of evading four-wave mixing, the optical fibers for WDM optical transmission systems are required to be unequipped with a zero-dispersion wavelength in their operating wavelength bands. In this context, a non-zero dispersion shifted fiber (NZDSF) has been developed, without any zero dispersion in the operating wavelength band. In general, NZDSFs are required to have even more complicated refractive-index (RI) profiles, than those of SMFs or DSFs, because they need to gear with additional requirements for a large effective core area (Aeff), a reduced dispersion slope, etc. to provide for high-density WDM (DWDM) optical transmission.
Complicated RI profile designs of NZDSFs accompany a propensity to induce minute glassmaking flaws in optical fibers, along with irksome process control.
Although NZDSFs are in use to cover a broad wavelength band including 1.55 μm, no hydrogen-proof treatment techniques were then disclosed to the public.
In the recent years, cables to shroud optical fibers have structurally been reviewed and improved. In fact, optical fiber cables are shifting in great numbers from a compound-filled type to a dry-core type which contains a water absorbent material in the cable instead of a filling compound. The filling-compound free cable fabrication is far less toilsome (not required to wipe clean the cables). Also the filled cables could hardly be enhanced in fire resistance, but filling-compound free (dry-type) cables can readily be attached with enhanced fire resistance. A sample dry-core type of optical fiber cable is described in U.S. Pat. No. 5,422,973.
The dry-core type contains a water absorbent material in the cable to block out lengthwise, water penetration, which contacts wet, swells and dams off the water. But then, the water absorbent material has an action to lead in ambient humidity (moisture), even without any cable damage or opening, and poses a threat of allowing the trapped wet (absorbed ambient moisture) to react with component metals inside the cable, where hydrogen ions emerge. Accordingly, even optical fibers for terrestrial cables need to be considered about their hydrogen-proof treatment.
For instance, U.S. Pat. No. 6,131,415 describes an optical fiber with a thought of hydrogen-proof performance (hydrogen resistance), and a technique for suppressing the hydroxyl-ion concentration in an optical fiber to reduce the transmission loss at 1385 nm. In particular, the present patent owner, Lucent Technologies, discloses in “Catalog Allwave,” certain information about a required design concept of hydrogen-proof performance (hydrogen resistance) of optical fibers for metropolitan use.
Moreover, U.S. Pat. Nos. 5,838,866 and 6,128,928 each describe an optical fiber with a thought of hydrogen resistance. Each fiber is designed to be equipped with hydrogen resistance by making the (inner) clad contain germanium to a degree to raise its refractive index, not in substance.
However, U.S. Pat. No. 6,131,415 remarks on no more than a technique for suppressing the loss increase at 1385 nm, arising out of absorption peaks of hydroxyl ions, without any remarks on the loss increase at 1520 nm, due to the absorption peaks of hydrogen molecules.
Moreover, none of the techniques in U.S. Pat. Nos. 6,131,415, 5,838,866 and 6,128,928 involve any additives to bring on substantial shifts in the refractive-index profile in each clad region. Thus, these three patents are expected to aim for characteristic improvements in “SMF” or the equivalent, but not NZDSF for WDM optical transmission systems.